hydrocarbons that have at least one triple bondbetween two carbonatoms, with the formula CnH2n-2. The alkynes are traditionally known as acetylenes or the acetylene series, although the name "acetylene" is also used to refer specifically to the simplest member of the series, known as ethyne(C2H2) using formal IUPACnomenclature.
alkanes, and to a lesser extent, alkenes, alkynes are unstable and reactive. Terminal alkynesand acetyleneare fairly acidic and have pKa values (25) between that of ammonia(35) and ethanol(16). This acidity is due to the ability for the negative charge in the acetylide conjugate baseto be stabilized as a result of the high s character of the sp orbital, in which the electron pair resides. Electrons in an s orbital benefit from closer proximity to the positively charged atom nucleus, and are therefore lower in energy. This can also be thought of in terms of electronegativity: electrons in an hybrid orbitalwith high s character reside closer to the nucleus. The closer proximity of the electrons to the nucleus allows an acetylinic carbon to have a greater amount of electronegative character. As a result, a proton is more easily removed from the carbon as electrons flow more willingly to a more electronegative atom.
A terminal alkyne with a
strong basesuch as sodium, sodium amide, "n"-butyllithium or a Grignard reagent, gives the anionof the terminal alkyne (a metal acetylide):
:2 RC≡CH + 2 Na → 2 RC≡CNa + H2
:RC≡CH + B → RC≡C− + HB+, where B denotes a strong base.
The acetylide anion is synthetically useful because as a strong
nucleophile, it can participate in C−C bond forming reactions.
It is also possible to form copper and silver alkynes, from this group of compounds
silver acetylideis an often used example.
The carbon atoms in an alkyne bond are
sp hybridized: they each have 2 p orbitals and 2 sp hybrid orbitals. Overlap of an sp orbital from each atom forms one sp-sp sigma bond. Each p orbital on one atom overlaps one on the other atom, forming two pi bonds, giving a total of three bonds. The remaining sp orbital on each atom can form a sigma bond to another atom, for example to hydrogen atoms in the parent compound acetylene. The two sp orbitals on an atom are on opposite sides of the atom: in acetylene, the H-C-C bond angles are 180°. Because a total of 6 electrons take part in bonding this triple bond is very strong with a bond strengthof 839 kJ/mol. The sigma bond contributes 369 kJ/mol, the first pi bond contributes 268 kJ/mol and the second pi bond is weak with 202 kJ/mol bond strength. The CC bond distance with 121 picometers is also much less than that of the alkenebond which is 134 pm or the alkane bond with 153 pm.
The simplest alkyne is
ethyne( acetylene): H-C≡C-H
Terminal and internal alkynes
Terminal alkynes have a hydrogen atom bonded to at least one of the sp hybridized carbons (those involved in the triple bond. An example would be
methylacetylene(1-propyne using IUPAC nomenclature).
Internal alkynes have something other than hydrogen attached to the sp hybridized carbons, usually another carbon atom, but could be a heteroatom. A good example is 2-pentyne, in which there is a methyl group on one side of the triple bond and an ethyl group on the other side.
The terminal Hydrogen atom is weakly acidic, and can be removed by a very strong base, to yield a salt. This property can be used as a chemical test to distinguish terminal alkynes from others, or the salt may be used to make larger alkyne molecules. A few drops of diamminesilver(I) hydroxide (Ag(NH3)2+ -OH or Ag(NH3)2OH)) solution are added to samples of a non-terminal alkyne and also a terminal alkyne. No reaction occurs for the non-terminal, but the terminal alkyne forms a characteristic white precipitate. This is the insoluble silver salt of the terminal alkyne:R-C≡CH + Ag(NH3)2+ -OH → R-C≡C- Ag+ + NH4+ + NH3 (R = general alkyl group)Warning: transition metal salts of terminal alkynes (metal; acetylides) can be explosive whendry.
Alkynes are generally prepared by
dehydrohalogenationof vicinal alkyl dihalides or the reaction of metal acetylides with primary alkyl halides. In the Fritsch-Buttenberg-Wiechell rearrangementan alkyne is prepared starting from a vinyl bromide.
Alkynes can be prepared from
aldehydes using the Corey-Fuchs reactionand from aldehydes or ketones by the Seyferth-Gilbert homologation.
Alkynes are involved in many
** addition of
hydrogento give the alkeneor the alkane
** addition of
halogens to give the vinyl halides or alkyl halides
** addition of
hydrogen halides to give the corresponding vinyl halides or alkyl halides
** addition of water to give the
carbonylcompound (often through the enolintermediate), for example the hydrolysisof phenylacetyleneto acetophenonewith sodium tetrachloroauratein water/methanol (scheme shown below) [Fukuda, Y.; Utimoto, K. "Effective transformation of unactivated alkynes into ketones or acetals with a gold(III) catalyst". " J. Org. Chem." 1991, "56", 3729–3731. DOI|10.1021/jo00011a058] or (Ph3P)AuCH3 [Mizushima, E.; Cui, D.-M.; Nath, D. C. D.; Hayashi, T.; Tanaka, M. "Au(I)-Catalyzed hydratation of alkynes: 2,8-nonanedione". " Organic Syntheses", Vol. 83, p.55 (2005). [http://www.orgsynth.org/orgsyn/pdfs/v83p0055.pdf Link] .] :
Diels-Alder reactionwith 2-pyroneto an aromaticcompound after elimination of carbon dioxide
Azide alkyne Huisgen cycloadditionto triazoles
Bergman cyclizationof enediynes to an aromaticcompound
Alkyne trimerisationto aromaticcompounds
** [2+2+1] cycloaddition of an alkyne,
alkeneand carbon monoxidein the Pauson–Khand reaction
** scrambling of alkynes in
alkyne metathesisto new alkyne compounds
** reaction with alkenes to butadienes in
nucleophilic substitutionreactions of metal acetylides
carbon-carbon bondformation with alkyl halides
nucleophilic additionreactions of metal acetylides
** reaction with
carbonylcompounds to an intermediate alkoxideand then to the hydroxyalkyneafter acidic workup in the Favorskii reaction.
* hydroboration of alkynes with
organoboranes to vinylic boranes
** followed by reduction by oxidation with
hydrogen peroxideto the corresponding aldehydeor ketone
* oxidative cleavage with
potassium permanganateto the carboxylic acids
* migration of the alkyne along a hydrocarbon chain by treatment with a strong base
Coupling reactionwith other alkynes to di-alkynes in the Cadiot-Chodkiewicz coupling, Glaser couplingand the Eglinton coupling.
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